1. Field of the Invention
This invention relates to a blast furnace system for the production of iron or ferroalloys utilizing an electric arc heater to superheat a reducing gas or fuel which is injected through the furnace tuyeres into the furnace charge or burden allowing for a reduction in the amount of coke which is normally used in the iron-making process.
2. Description of the Prior Art
The blast furnace is the world's primary apparatus for the reduction of iron-bearing materials, such as iron ore, into pig iron. Blast furnaces represent very old technology and have been the subject of major scale-up efforts over the past 20 years. For this reduction process coke is combusted in the furnace with preheated air to provide heat and a reducing gas for the reduction reaction involving the iron-bearing material.
Existing blast furnace systems employ a refractory lined vertical shaft furnace having gas uptakes attached to the top of the shaft, a gas separator, blast air heating stoves, and injection tuyeres. The upper, middle and lower regions of the shaft are known as the stack, bosh and hearth, respectively. In operation, a mixture of iron-bearing materials, coke and flux, which is known as burden, is introduced at the top of the furnace. This burden slowly descends against an upwardly rising flow of reducing gas. Preheated blast air or wind is introduced into the charge through water-cooled tuyeres combusting the coke to produce carbon monoxide and heat. The carbon monoxide reduces the iron-bearing materials into iron which is then melted by the heat liberated by the combustion of the coke and a portion of the heat contained in the blast air. The melted iron and slag which is produced by the reduction reactions are collected in the hearth from which they are removed via outlets known as iron notches and slag notches, respectively. Excess reducing gas and heat rises upwardly through the stack to cause the preheating and pre-reduction of the burden contained therein. This reducing gas and heat exits the top of the furnace and enters the gas uptakes. At this stage these gases are typically referred to as off-gases. These off-gases are then directed into a gas separator for the removal of substantially all entrained particulate materials. The cleaned off-gases are sent to the blast air heating stoves where they are combusted to heat the blast air which will be injected through the tuyeres. This blast air is heated in the stoves to a temperature of approximately 1000.degree. C. A more detailed account of the construction and operation of conventional blast furnace systems may be found in the handbook entitled The Making, Shaping and Treating of Steel, 9th ed. Pittsburgh, United States Steel Corporation, 1971, pp. 422-472.
Operation of conventional blast furnaces requires the use of large amounts of coke per ton of hot metal produced. In the United States a typical blast furnace requires about 1150 pounds of coke per ton of hot metal with this coke being produced from costly high-grade metallurgical coal. Because of the expense and environmental problems associated with the production of coke, it would be advantageous to have a blast furnace which can produce pig iron using a lower amount of coke per ton of hot metal produced.
Work is currently underway to effect a significant reduction of the blast furnace coke rate by the injection of electric arc superheated reducing gas through the blast furnace tuyeres. In a paper entitled "Operation of a Blast Furnace with Superhot Reducing Gases" by N. Ponghis, R. Bidal, and A. Poos, Department of Ironmaking and Reduction Centre de Recherches Metallurgiques, Liege, Belgium, presented at the AIME Conference in Detroit, Mich., on Mar. 26, 1979, a single tuyere experimental blast furnace utilizing tuyere injection of a reducing gas superheated by an electric arc furnace, is described. There the electric arc furnace or plasma furnace was used to produce a superhot reducing gas having a temperature above 2000.degree. C. which was injected directly into the main tuyere allowing for a reduction in the coke rate of up to 75%. The reducing gas was obtained by reforming natural gas with either air or CO.sub.2 in the plasma furnace which was located at the nose of the tuyere. Thus, it was shown that a significant reduction in the coke rate of a blast furnace could be obtained by the injection of a reducing gas superheated by an electric arc furnace. Another benefit noted by the authors was that the silicon content of the iron could be easily controlled by varying the temperature of the injected superheated reducing gas. They noted that the response of the silicon to such variations was practically instantaneous thus eliminating the long dead and response times normally found in a conventional blast furnace. In U.S. Pat. No. 4,247,732 issued Jan. 27, 1981 and entitled "Method and Apparatus for Electrically Firing An Iron Blast Furnace" an arc heated mixture of a gas containing carbon monoxide is injected into a vertical shaft furnace for reducing iron ore. This furnace also incorporates the benefit of reduced coke consumption.
While these two furnaces have demonstrated that the coke rate can be substantially reduced by the use of an electric arc heater to superheat a reducing gas, the electricity used to power the electric arc heater was provided from a conventional source of power. Therefore, it would be advantageous if the electricity used by the electric arc furnace could be produced by recouping some of the energy contained in the products of the reduction reactions exhausted from the blast furnace. This would eliminate the expensive requirement of purchasing utility generated electricity for the operation of the electric arc heaters.